A direct esterification method between dicarboxylic acid and diol in the presence of a catalyst, or a method for producing an aliphatic polyester resin having high degree of polymerization by producing an ester low polymer by an ester exchange reaction between an alkyl ester of dicarboxylic acid and diol, and while conducting an ester exchange reaction of the ester low polymer under heating and reduced pressure, distilling away the formed diol from a reaction system is known from a long time ago and employed as an economically advantageous production method of an aliphatic polyester resin.
However, in the conventional production method of an aliphatic polyester resin, an aliphatic polyester resin of high degree of polymerization having practically sufficient strength has not been obtained for the reason that thermal stability of the aliphatic polyester resin is low and the resin is thermally decomposed during polymerization reaction, thereby inducing lowering of a molecular weight. In view of the above, to obtain an aliphatic polyester resin having sufficient strength, various creative efforts are made in its production method.
For example, a method of increasing melt viscosity of an aliphatic polyester resin by conducting melt polymerization using an organic alkoxy metal compound such as tetrabutyl titanate as a catalyst, and mixing diisocyanate and/or diphenyl carbonate as a chain extender, thereby increasing the degree of polymerization of an aliphatic polyester is proposed (Patent Document 1).
Furthermore, a method of crosslinking aliphatic polyester resins with each other by mixing, as a crosslinking agent, trifunctional hydroxycarboxylic acid in an amount of from 0.5 to 5% by mole, or tetrafunctional hydroxycarboxylic acid in an amount of from 0.1 to 3% by mole, to dicarboxylic acid is disclosed as a production method of increasing melt viscosity of an aliphatic polyester resin (Patent Documents 2 and 3). However, properties of the aliphatic polyester resin produced are insufficient even by those methods, and creative efforts to increase a molecular weight of the aliphatic polyester resin are made by further mixing diisocyanate at the latter-stage of polymerization reaction (Patent Documents 4 to 6).
Furthermore, a method of increasing a molecular weight of an aliphatic polyester resin using the above-described crosslinking agent without using a diisocyanate compound, a diphenyl carbonate compound and the like generally used as a chain extender is proposed in recent years (Patent Documents 7 to 10).    Patent Document 1: JP-A-4-189822    Patent Document 2: U.S. Pat. No. 3,079,717    Patent Document 3: U.S. Pat. No. 3,079,718    Patent Document 4: JP-A-5-178955    Patent Document 5: JP-A-5-178956    Patent Document 6: U.S. Pat. No. 3,072,808    Patent Document 7: JP-A-8-259679    Patent Document 8: JP-A-2005-2331    Patent Document 9: JP-A-2006-274252    Patent Document 10: JP-A-2006-274253